Among the common table vegetables available for human consumption is the table beet. Beets are part of the diet of many people. While there has been some development of new varieties of beet, the genetic potential of table beets, both in variety and production, is not close to being fully explored.
Another trend in modern vegetable development is the creation of varieties of plants which have enhanced concentrations of natural plant metabolites. This is done for two reasons. One reason is to make eating vegetables of increased nutritional value. The other reason is to use the vegetable as a mechanism to produce the metabolite in useful quantities so that the can be extracted for nutritive purposes, for additives to other foods, or potentially even for therapeutic or prophylactic purposes. Among the plant metabolites which are attractive for enhancement are pigment molecules, many of which are also associated with the dietary benefits that humans receive when eating pigmented vegetables.
Table beets are generally available in the United States in two colors, red and golden. The color of beets is determined by a class of pigment molecules known as betalains. Betalains are water-soluble, vacuolar pigments produced by select fungi and plants within select families of the Order Caryophyllales, including the table beet (Beta vulgaris subsp. vulgaris). Betalains can be differentiated from anthocyanins by their nitrogenous composition and biosynthesis. Betalains and anthocyanins have a mutually exclusive production in plants. Plants that produce betalains do not produce anthocyanins and vice versa. Anthocyanins arise from a condensation between three malonyl-CoA molecules while betalains are created from the condensation of a primary or secondary amine (cyclodopa glucoside or 4,5-secodopa) with betalamic acid resulting in the structure of a 1,2,4,7,7-pentasubstituted 1,7-diasaheptamiethin system.
Betalains are classified into two major pigment groups: betacyanins (BC) and betaxanthins (BX). Betacyanins are produced from a condensation of betalamic acid with cyclodopa while betaxanthins result from a condensation reaction of betalamic acid with specific amino acids. Betacyanins (Amax 540 nm) produce a red-violet hue; betaxanthins (Amax 480 nm) produce a yellow hue. The betacyanins differ from the betaxanthins by the conjugation of a substituted aromatic nucleus to the 1,7-diasaheptamethinium chromophore, which is present in betacyanins. The ratio of the two pigments contributes to the final color of the plant tissue. Beets in which the concentration of betacyanins is high enough appear red in color, the red color produced by the betacyanins masking the concentration of betaxanthins.
While there is significant variation in pigment content in beet genotypes, in commercial beet varieties currently on the market, betaxanthins constitute only 20-30% of the total pigments contents of the beets. Efforts have been made before to produce beets which have enhanced levels of betacyanins. U.S. Pat. No. 6,353,156 describes a high pigment red table beet with enhanced levels of betacyanins produced in its tissues. Those beets were bred for generalized high pigment levels, and being red beets, produce enhanced levels of both betacyanins and betaxanthins.
Significant effort has been made in recent years to improve the nutritional content of table foods by increasing the amounts of metabolites in the food thought to be of nutritional importance. Betalains are one such metabolite, being thought to be significant antioxidants. However, no presently commercially availably beet varieties are high in betaxanthins without concurrent high concentrations of betacyanins. While there is one commercial variety of yellow table beet currently sold for cultivation in the U.S., known as Burpee's Golden, and some essentially derived cultivars, the variety and the cultivars do not have high levels of betaxanthins, the yellow color being simply a reduction in total pigment in the variety. The cultivars also tend to have poor seed quality. There are no commercial varieties of golden beet with high levels of betaxanthins.